1. Industrial Field of the Invention
The present invention relates to an electrically conductive paste and a ceramic multi-layered substrate formed by using the electrically conductive paste. In particular, this invention relates to an electrically conductive paste and a ceramic multi-layered substrate which can be sintered at a temperature of 1000.degree. C. or lower.
2. Description of the Related Art
A ceramic multi-layered substrate has been used as a circuit substrate when forming an integrated circuit, a high frequency electronic circuit, a hybrid circuit or the like. Usually, a ceramic multi-layered substrate may be obtained by forming within a ceramic body or on the surface thereof either a material layer consisting of an electrically conductive paste or via hole connecting portions, followed by a sintering treatment of the ceramic body. In fact, such a sintering treatment will produce a ceramic sintered body and electrically conductive sintered portions within the ceramic sintered body and/or on the surface thereof.
Such a ceramic multi-layered substrate has been used in various electronic instruments. However, in recent years, in order to meet the demands of high frequency and high speed required by various electronic instruments, the electrically conductive sintered portions of a ceramic multi-layered substrate should be made of a material containing as its main component a low resistance conductor such as silver, gold, or copper. Further, the electrically conductive sintered portions are formed by an electrically conductive paste, and are sintered at the same time when the ceramic body is sintered. For this reason, if the electrically conductive paste contains a low resistance conductor such as silver, gold or copper, a simultaneous sintering treatment can be carried out to simultaneously treat both the ceramic body and the electrically conductive paste. Therefore, the ceramic composition to be contained in the ceramic body can be a glass ceramic capable of being sintered at a relatively low temperature such as a temperature of 1000.degree. C. or lower.
Further, a ceramic multi-layered substrate is usually required not only to have a high density, many functions and high reliability, but also to have high transverse strength against possible breaking. However, since the above glass ceramic capable of being sintered at the same time as the electrically conductive paste containing silver, gold or copper is being sintered often contains a large amount of binder in order to be successfully sintered at a low temperature, the finally formed ceramic body has only a low transverse strength. In order to solve that problem, it has been suggested that some measures be taken to precipitate a kind of crystal during a process of sintering treatment of the above glass ceramic.
On the other hand, the process of manufacturing the above ceramic multi-layered substrate comprises steps of preparing a plurality of green sheets each containing a ceramic composition, printing an electrically conductive paste on to a specific pieces of green sheet, laminating one green sheet upon another so as to form a ceramic body, and sintering the ceramic body. During the sintering treatment, warping and deformation will occur in the thus obtained ceramic multi-layered substrate due to a difference in coefficient of contraction between the green sheets and the electrically conductive paste included in the ceramic body.
In order to solve the above problem, it has been suggested to use a material having only a small coefficient of contraction difference between the green sheets and the electrically conductive paste (such as that disclosed in Japanese Examined Patent Publication No. 3-26554), or to use an electrically conductive paste having a higher contraction ending temperature than a contraction ending temperature of the green sheets (such as that disclosed in Japanese Unexamined Patent Publication No. 2-94595). Further, it has been proved useful to control the particle diameter of metal powder contained in the electrically conductive paste, and to add a kind of frit, different kinds of metals having different melting points and a kind of metal oxide in the electrically conductive paste, thereby to effectively control the contraction of the electrically conductive paste.
When, in order to improve transverse strength of the ceramic sintered body consisting of a low temperature sinterable glass ceramic, a kind of crystal is precipitated in the sintered ceramic body, a severe contraction will occur in the sintered ceramic portions at a temperature near the crystal precipitating temperature. Further, flowability of the glass contained in the ceramic portion will be lost. Because of this, when the ceramic portion undergoes contraction, the electrically conductive paste portions will only contract a little, thus producing a contraction difference between the electrically conductive paste and the ceramic portions. As a result, if these contractions are in a mutually sticking relation, a sort of warping phenomenon will occur, causing an increased deformation in a finally obtained ceramic multi-layered substrate.
Usually when a silver metal powder alone is used as the electrically conductive component in the electrically conductive paste, even if the particle size of the metal powder and its specific surface area are changed, the electrically conductive paste will begin to contract at a temperature of about 300-750.degree. C. As a result, even if the contraction curve is very gentle, when the ceramic portion undergoes a contraction, the electrically conductive paste portions will also begin to contract.
Consequently, as has been disclosed in Japanese Examined Patent Publication No. 3-26554, even if what is used are some specifically selected green sheets and electrically conductive paste, one of which has a contraction coefficient which differs little from that of the other, it is still difficult to prevent deformation of the finally obtained ceramic multi-layered substrate. This problem is caused due to a fact that when the ceramic portion undergoes contraction, the electrically conductive paste portions will also begin to contract, while one of them usually has a contraction curve that is different from that of the other.
On the other hand, when a ceramic material to be used is a glass ceramic containing a large amount of amorphous glass component, the glass will have a high flowability even after the ceramic portion has completed its contraction. As a result, a deformation possibly caused due to the above-mentioned contraction difference will be alleviated due to the flowing of the glass, thus prohibiting warping or deformation in the finally obtained ceramic multi-layered substrate. However, since a glass ceramic containing a large amount of amorphous glass component has a low transverse strength, crystal precipitation is needed in order to obtain a high transverse strength, but when a crystal has been precipitated, the flowability of the glass will become low. As a result, it is difficult to alleviate deformation caused by a contraction difference between the ceramic portions and the electrically conductive paste, making it impossible to avoid warping and deformation in the finally formed ceramic multi-layered substrate.
Even if we can try to use an electrically conductive paste having a higher contraction ending temperature than that of the green sheets, as described in Japanese Unexamined Patent Publication No. 2-94595, contraction of the ceramic portions and contraction of the electrically conductive paste will occur at the same time, with the contraction curves thereof being different from each other, resulting in a deformation caused due to the contraction difference between the two kinds of materials. In fact, such kind of deformation is impossible to be alleviated if a crystal is to be precipitated for obtaining a high transverse strength. Consequently, it is not expected that an effect that the above-mentioned warping or deformation can be alleviated or cured.
Further, if an electrically conductive paste contains a kind of frit, different kinds of metals having different melting points and a kind of metal oxide in an electrically conductive paste, it is possible to effectively control the contraction of the electrically conductive paste. In order for the electrically conductive paste to have a contraction curve which is coincident with that of the ceramic portions, it is necessary to add a large amount of the above substances in the electrically conductive paste. As a result, the electric resistance of the electrically conductive sintered portions obtained by sintering the electrically conductive paste will be decreased. If such a product is used as a ceramic multi-layered substrate to form a high frequency electric circuit, there will be a large conductor loss, hence bring about a deterioration in the performance of the electric circuit.